A pressure sensor having liquid in a pressure sensing chamber as a pressure transmitting member is proposed in JP-A-2002-98607. The pressure sensor has a case in which the sensing chamber is provided. A pressure sensing element is arranged in the sensing chamber and a diaphragm is fixed to the case such that it covers an opening the pressure sensing chamber.
A cross-sectional view around the sending chamber is shown in FIG. 8. A first case 110 has a recess 111, a space of which is used as a sensing chamber 140. Liquid 115 that is a pressure transmitting member, such as oil, is confined in the sensing chamber 140. A metal diaphragm 134 for receiving pressure is fixed to the first case 110 such that it seals the sensing chamber 140 and has contact with the liquid 115. The diaphragm 134 receives pressure applied in the direction indicated with a white arrow.
A pressure sensing element 120 is arranged in the sensing chamber 140 such that it receives pressure from the liquid 115. The pressure received by the diaphragm 134 is transmitted to the sensing element 120 via the liquid 115. The sensing element 120 is electrically connected with terminals 112 via wires 113. The sensing element 120 outputs signals according to levels of pressure applied by the liquid 115 and the signals are transmitted to an external device via the wires 113 and the terminals 112.
An O-ring 142 and a welding ring 135 arranged around the sensing chamber 140 between the diaphragm 134 and the first case 110. The O-ring 142 is placed adjacent to the first case 110. It seals the sensing chamber 140. The welding ring 135 is a holding member for supporting the diaphragm 134, which is thin and requires a support. The diaphragm 134 and the welding ling 135 are welded to a second case 130 having a pressure intake hole 132. The second case 130 is fixed integrated to the first case 110 by swaging. The diaphragm 134 is pressed against the first case 110 via the O-ring 142 and the welding ring 135 with a force applied when the second case 130 is swaged fixed to the first case 110. As a result, the sensing chamber 140 is sealed.
The liquid 115 may leak when the O-ring 142 is damaged, the O-ring 142 is not tightly fitted due to faulty swaging, or welding of the diaphragm 134 or the welding ring 135 is not properly welded. It may leak when sealing surfaces of the diaphragm 134, the O-ring 142, the welding ring 135 are rough or have scars, burrs, or dents. It may leak when foreign substances are present in the sealing areas. It may leak through a gap between the first case 110 and the terminal 112 when a sealant 114 that seals the gap is broken or removed.
The diaphragm 134 becomes freely movable when the liquid has leaked. As a result, the diaphragm 134 moves toward the sensing element 120 and the wires 113 when a high level of pressure is applied to the diaphragm 134 though the pressure intake hole 132. The diaphragm 134, the sensing element 120, and the wires 113 are electrically shorted at the worst case. The diaphragm 134 a thin plate and therefore it is not steady. Moreover, it is an electric conductor. Thus, electrical shortage occurs among the diaphragm 134, the sensing element 120, and the wires 113.